Add support for SRXL2 as ESC protocol

[RobertoD91]

Current Behavior

I installed INAV on an E-flite Super Timber 1.7, which includes a "Spektrum Avian Smart Lite ESC".
This ESC can be controlled using:

• classic PWM, without telemetry or reverse
• "Smart Throttle", which is SRXL2 and includes telemetry (battery, RPM, current, etc.) and motor reverse

This amazing project dgatf/msrc#152 discovered that it uses SRXL2 and programmed an RP2040 to act as a converter, allowing this and other ESCs to be used with non-Spektrum receivers.

Desired Behavior // Suggested Solution

Adding SRXL2 as an ESC protocol in iNav

Who does this impact? Who is this for?

Adding SRXL2 as an ESC protocol would allow the use of Spektrum ESCs without an external RP2040.
This could be very useful for flight controllers like the RadioMaster Nexus.

Additional context

This is not critical; it may be a "nice to have".
Thank you for this awesome work :)